Summary: The NHGRI intramural program has expanded research efforts to integrate virus vectors into adult hematopoietc stem cells (HSC) to combat HIV infection. The HSC is the ultimate progenitor of all peripheral blood cells, including the CD4+ lymphocytes that are infected by HIV and the macrophages that can sequester HIV for long periods of time in tissues. If an anti-HIV element can be introduced into HSC, it will be passed along to all of the progeny of that HSC, ensuring the continuous, life-long production of HIV resistant cells. If this therapy were completely successful, this treatment would permanently protect the patient from HIV spread and no further treatments would be required. The successful modification of HSC by viral vectors has three important steps. First the viral vector must bind to a specific receptor on the surface of the HSC. We have determined that the receptor for the FeLV-C retrovirus is plentiful on HSC, unlike the conventional virus receptors, which are not present on HSC. We have demonstrated high levels of gene transfer into human HSC using retrovirus vectors with these new envelopes using mouse and sheep xenograft models. In the coming year, we will extend these studies to include pseudotyping lentivirus vectors that carry anti HIV genes. The second important step in the modification of HSC requires the vector to become integrated into the DNA of the target cell. Because of new safety concerns regarding insertional leukemogenesis in a gene therapy trial, NHGRI intramural researchers are comparing the integration sites of lentivirus and oncoretrovirus vectors. We have built up an extensive library of viral integrations in CD4+ lymphocytes and are correlating these with expression changes. We are also studying several genes that regulate hematopoietic cell cycle progression. The final important step for successful modification of HSC with anti-HIV elements is that the anti-HIV elements have to be produced in the mature progeny of the HSC at all times so that the cells are always prepared to interrupt HIV infection. Many groups have shown that viral vectors can become silenced over time and stop making the critical elements. To combat silencing, NHGRI intramural investigators are developing and evaluating different virus vectors that contain genetic elements to prevent silencing. Among the vectors under development are vectors containing insulator elements that prevent the type of changes associated with gene silencing. Novel insulators are being incorporated into vectors to evaluate gene silencing in mouse models. The combination of these three initiatives should significantly improve the prospects for successful HSC based AIDS gene therapy. It is anticipated that the results of these studies can be combined into a clinical trial for AIDS gene therapy in the near future. NHGRI has initiated a genetic approach to AIDS that takes full advantage of the strengths of the intramural NHGRI clinical research program. NHGRI clinical researchers study inherited immune disorders building knowledge that will bear on the ultiumate production of HIV vaccines and provide an understanding of how HIV deactivates the immune system. The NHGRI Genetic Counseling program will initiate an effort to counsel and educate individuals with AIDS. The Genetic Counseling program has completed an extensive study of the most effective counseling techniques for families with inherited immune disorders. The NHGRI AIDS initiative will compare counseling approaches that have been shown to be effective in inherited immunodeficiency diseases with counseling for AIDS families. Such psychological studies are expected to reveal whether different approaches are necessary to help families cope with AIDS and its consequences.